Flow meter

ABSTRACT

This invention relates to a flow meter of a fluidistor oscillator type, which has its principal use when measuring the ventilation of a moving human being or animal. Thanks to the special design of the flow meter it can be made small, light and portable at the same time as the pressure drop in it is minimal.

This invention relates to a flow meter of a fluidistor oscillator type.The object of the invention is to make a flow meter for above allrespiratory measurements. The flow meter according to the invention hasits principal use when measuring the ventilation of human beings andanimals when the measurement is carried out on a moving object. In thesecases it is of particular importance that the flow meter is small, lightand portable at the same time as the pressure drop in the meter isminimal.

At present when making these measurements dry gas meters and gas tightbags containing 10-20 l (i.e. Douglas bags) are used into which theexhaled air is led at the same time as the collecting time is measured.Both measuring devices are bulky and heavy compared with the presentinvention. Possible solutions with flow meters of other types than theone of the invention are also cumbersome and heavy.

The invention relates to a special design for a flow meter of thefluidistor type. A fluidistor consists of an inlet duct and two outletducts that form a certain angle with each other. Between the inlet ductand the outlets there is on each side a control inlet. If one of thecontrol inlets is closed a vortex arises at its orifice and thereby apartial vacuum that directs the flow to the outlet on this side. If thenthis control inlet is opened and the other closed, the flow is in thesame way directed to the other outlet. By joining the two control inletsthe flow will switch between the two outlets. With a suitabledimensioning the switching frequency is directly proportional to theflow.

To use a fluidistor flip-flop as a flow meter is previously known fromthe Swedish Published Patent Application No. 349,146, and the U.S. Pat.Nos. 3,238,960 and 3,802,283.

The aimed qualities of the flow meter are according to the inventionachieved by giving the flow meter the special design that is evidentfrom the following claim.

A suitable embodiment of the flow meter according to the invention willnow be described with reference to the attached drawing where FIG. 1shows a section through a flow meter and

FIG. 2 a section along A--A in FIG. 1.

The flow meter comprises an inlet 1, two control inlets 5 and 6, acontrol duct 7 and two outlet ducts 8 and 9 located in one and the sameplane. In this way the flow meter can be made small.

The flow meter is designed as a box with flat top and bottom. Except bytop and bottom the inlet is limited by two guide surfaces 3 and 4, whichgive a wedge formed inlet. The inlet can thus be made short. The anglesbetween the guide surfaces 3 and 4 and the symmetry plane A--A of theflow meter in FIG. 1 are each suitably between 40° and 50°. Further theorifice of the inlet is covered by a net 2. In this way is obtained thatwind at outdoor use does not disturb the function of the meter. Withoutthe net the inlet would have to be considerably longer. The two controlinlets 5 and 6 are located on each side of the connection between theinlet 1 and the outlet ducts 8 and 9. The control inlets areinterconnected through the control duct 7, which runs circularly aroundthe two outlet ducts 8 and 9. In comparison with the usual design with amore cornered control duct a better signal-to-noise ratio is achieved.The design also helps to make the meter small. The outlet 10 isperpendicular to the direction of inflow and the plane that is formed bythe inlet 1, the control inlets 5 and 6, the control duct 7 and theoutlet ducts 8 and 9 and is located in the centre of the circle that isformed by the control duct 7. The outlet takes up all the space betweenthe outlet ducts 8 and 9 so that the pressure drop in the meter will beminimal. A connecting duct between the rear ends of the two outlet ducts8 and 9 is fitted with a crest 11, which helps to guide the flow to theoutlet 10. The oscillations are detected by the pressure waves that areformed in the control duct by means of a transmitter 12 and a reciever13 for ultrasonics placed in the endpoints of the control duct anddirected towards a point 14 on the symmetry plane of the flow meterlocated from the inlet somewhat beyond the centre of the control ductand on one half of the height between the top and the bottom of themeter. By aiming the transmitter and reciever in this way the ultrasonicsignal will go through the control duct with wall reflections.

In one embodiment the flow meter had the folowing data: length 110 mm,width 85 mm, height 44 mm, weight 80 g, pressure drop 4 mm H₂ O at 100l/min and 28 mm H₂ O at 200 l/min, measuring range 16-250 l/min anderror in measurement less than 2% RMS of the indicated value over theentire measuring range.

By changing the geometry in a linear way the measuring range can bemoved upwards or downwards.

We claim:
 1. A flow meter of a fluidistor oscillator type with twointerconnected control inlets located on each side of the connectionbetween an inlet and two outlet ducts and with the inlet, the controlinlets, the control duct and the outlet ducts located in one and thesame plane and in which the oscillation is detected by the pressurewaves that are formed in the control duct, characterized in that theinlet is fitted with a net over the orifice and is limited by twocontrol surfaces, which give a wedge formed inlet, that the control ductruns circularly around the two outlet ducts, that the outlet isperpendicular to the direction of inflow and the plane that is formed bythe inlet, the control inlets, the control duct and the outlet ducts andis located in the centre of the circle that is formed by the controlduct, that a connecting duct between the rear ends of the two outletducts is fitted with a crest, which helps to guide the flow to theoutlet, and that the oscillations are detected by means of a transmitterand a receiver for ultrasonics placed in the endpoints of the controlduct and directed towards a point on the symmetry plane of the flowmeter located from the inlet somewhat beyond the centre of the controlduct and on one half of the height between the top and the bottom of themeter.